One of the problems at the root of the present invention concerns the pollution generated by the operation of these turbines. More precisely, nitrogen oxides (NOx) and carbon monoxide (CO) emissions must be reduced because they are the most harmful to the environment.
Furthermore, rather stringent standards are in force or will come into force in most industrialized countries.
Nitrogen oxides (NOx) are mainly thermal nitrogen oxides that form at high temperature, i.e. above 1700 K in gas-turbine combustion chambers where the fumes have residence times generally ranging between 2 and 10 milliseconds.
Carbon monoxide (CO) forms at a lower temperature (&lt;1600 K) by incomplete combustion of the fuel.
The optimum temperature range for reduced NOx and CO emissions is thus between about 1650 K and 1750 K. FIG. 1 illustrates, by means of (CO and NOx) curves, the respective carbon monoxide and nitrogen oxides emissions as a function of the temperature T (in K) under the operating conditions of a gas-turbine combustion chamber.
NOx and CO emissions are thus directly linked with the air-fuel mixture strength in the combustion chamber, i.e. the ratio of the flow of air to the flow of fuel. Given that the air-fuel ratio of the mixture must be imposed if one wants to operate within a certain temperature range, such as that mentioned above, the adiabatic flame temperature of the mixture will approximately vary proportionally to the mixture strength.
Conventionally, as it is well-known, the flow of fuel is the only parameter allowing to control the operating conditions of the turbine. For a given flow of fuel, the flow of air is therefore perfectly set to a value depending only on the characteristics of the machine and in particular on the cross-sections of flow in the furnace. The mixture strength is thereafter totally determined.
However, the mixture strength range allowing to respect the temperature range defined above does not always correspond to the mixture strength imposed by the characteristic curve of the machine.
Several concepts can be envisaged to solve this problem.
One of them consists in carrying out a combustion in several stages, ignited successively. This known solution is illustrated by FIG. 2 that shows a combustion chamber having a pilot stage followed by two other stages having each an air inlet and an inlet for a fuel such as natural gas for example. Combustion then has to be performed in each stage successively and according to the total power required. The pilot combustion is carried out whatever the speed. This solution theoretically allows to obtain acceptable mixture strengths in the ignited stages, for each engine speed, if a sufficient number of stages is available. The major drawback is that it requires a complex fuel delivery circuit, hence reliability, control and cost problems.
Another concept allowing to obtain combustion chambers operating in a determined temperature range consists in equipping it with a series of shutters, clappers or other shutoff means allowing to control the flow of air in the furnace. Of course, control and actuation of such elements is complex and delicate to implement. This equipment is furthermore costly.